44-74861tch_microatrobjective.pdf

7/27/2019 44-74861TCH_MicroATRObjective.pdf

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Introduction

Attenuated total reflectance (ATR)

techniques are well established

in FT-IR spectroscopy for the di-rect measurement of solid and

liquid samples without sample

preparation. The technique re-

quires good contact between the

sample and a crystal made from

a material which transmits IR

radiation and has a high refrac-

tive index. When the IR beam

enters the crystal at the critical

angle, internal reflection occurs.

At each reflection, IR radiationcontinues beyond the crystal

surface and enters the sample.

The depth of penetration de-

pends on the refractive indices

of the crystal and the sample,

the angle of incidence of the

beam, and the wavelength of the

IR radiation. For a germanium

crystal, the penetration depth

(for a sample of refractive index1.4) between 3000 and 1000 cm-1

ranges between approximately

0.2 and 0.6 m, allowing good

spectra to be collected from

optically thick, non-reflecting

samples.

The multimode objective

For the analysis of microsamples

using ATR, a small crystal allow-

ing a single reflection is incorpo-rated into the cassegrain objective

of the PerkinElmer microscopes.

This forms the unique PerkinElmer

multimode objective (Figures 1

and 2), in which the crystal has

two on-axis positions, raised and

Applications and Design

of a Micro-ATR Objective

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No re-alignment required

Spring-loaded crystal

100 m contact area

Applications

Thick samples

Non-reflecting samples

Contamination studies

Depth profile studies

Contamination spots on paper

Paintings

Key Features

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FT-IRSPECTROSCOPY

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lowered. When the crystal is raised,

the sample can be viewed, brought

into focus and centered in the field

of view. The crystal is then simply

lowered to contact the sample, and

a spectrum can be collected. Impor-

tantly, when raised, the crystal does

not obscure the optical path throughthe cassegrain. This allows the other

modes of data collection, transmis-

sion and external reflectance, as

well as viewing the sample, to be

carried out without either removing

the micro-ATR assembly or switch-

ing to another objective. Permanent

alignment of all components in the

multimode objective is therefore

maintained ensuring reproducible

and accurate measurements.Germanium is usually the crystal

of choice, as the short penetration

depth provided by this material

gives the widest range of possible

applications, including carbon

filled materials. Silicon is also

available. The contact area of the

crystal is a circular section with

a diameter of 100 m, although

smaller samples can be measured

using the apertures of thePerkinElmer microscopes.

Figure 1 shows that the crystal mount

is spring-loaded. This ensures a high

contact pressure at the sample, while

at the same time, safeguarding against

damage by ensuring that the crystal

will automatically lift when exces-

sive force is applied. Inconvenient

contact alert or pressure sensing de-vices are not necessary for use with

this design.

Applications

Thick samples

In combinatorial chemistry, organic

compounds are synthesized while

bound to a base resin formed from

polymer beads. Typically, beads have

a diameter of 100 m, which is toothick for meaningful transmission

measurements, as many bands in the

spectrum will totally absorb. However,

using the micro-ATR objective, use-

ful spectra can be obtained from a

single bead. In the example shown

in Figure 3, a compound containing

an ether linkage and a carbonyl

group was synthesized on a modi-

fied polystyrene resin. Spectra of

the beads before and after the reaction

are shown, along with a difference

spectrum which shows a reduced

contribution from the resin.

Non-reflecting samples

Carbon-filled rubbers and polymers

present an additional problem if they

are to be analyzed without any sam-

ple preparation. As well as being

optically thick, they reflect little

IR radiation which rules out direct

external reflectance as a viable

sampling method. However, with a

germanium crystal, micro-ATR will

quickly provide an excellent spec-

trum. Figure 4 shows the spectrum

of a fragment of carbon-filled nylon.

Contamination studies

The micro-ATR objective is an

essential tool for the analysis of

contamination in final products and

packaging materials, as no sample pre-paration is required. Contamination

in paper presents a particular chal-

lenge as it can appear in many forms,

including surface discoloration and

embedded or surface particles. In

many cases, it is impossible to cleanly

remove the contamination from the

paper without adhering fibers.

In Figure 5, spectra from a paper

sample are shown. Small areas of

the sample had a different surface

texture, and had a very slight blue

tint. Spectra from bulk and blue

areas reveal that the blue area has a

concentration of calcium carbonate

filler on the surface.

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Figure 1. A schematic of the multimode objective

Figure 2. The multimode objective.


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fifth spectrum, an additional broad

band is seen at ca. 1400 cm-1, along

with a much sharper band at ca.

870 cm-1. These bands indicate that

there is a lower layer of calcium

carbonate onto which the pigment

has been applied.

intensity of the spectrum in the re-

gion 1050 - 1000 cm-1, where there

are strong bands from the ochre,

increases as the sampling depth

increases. This is likely to be due

to a greater contribution from the

ochre deeper into the sample. In the

Depth profile studies

When working with soft materials, it

is sometimes necessary to improve

the contact between the sample and

the crystal. This can be achieved

by slightly raising the sample stage

to increase the contact pressure. As

previously discussed, there is no

danger of either damaging the crystal

or the cassegrain objective, as the

crystal is spring-loaded. For many

softer samples, once good contact

has been made, it is possible to

push the crystal further into the

sample, and spectra may be obtained

which contain information about

sub-surface layers.

Contamination spots on paper

Spectra from a contamination spot on

a paper sample are shown in Figure 6,

along with a spectrum from the

bulk paper. The contaminant has a

carbonyl band at 1710 cm-1 which

becomes relatively more intense

than the band at 1650 cm-1 (from the

bulk paper) as the sampling depth is

increased. Similar changes are seen

in the region 1300 - 1100 cm-1. The

fact that the spectrum becomes lesslike the bulk spectrum indicates

that the carbonyl containing con-

taminant is mainly embedded under

the surface of the paper.

Paintings

A fragment from an ancient wall

painting was analyzed in a similar

way. Figure 7 shows five spectra

from different depths, and a refer-

ence spectrum, also measured by

micro-ATR, of a red ochre pigment

used in the painting. The top four

spectra show bands from the red

ochre and a surface varnish. The

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Figure 3. Micro-ATR spectra of organic substituents on resin beads. Collection time:20 sec. Resolution: 8 cm-1. Germanium crystal.

Figure 4. Micro-ATR spectrum of a fragment of carbon-filled nylon. Collection time:15 sec. Resolution: 8 cm-1.

Figure 5. Micro-ATR spectrum of a paper surface. Collection time: 15 sec.Resolution: 8 cm-1.


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Conclusions

A multimode objective, which allows

viewing and all IR modes of oper-

ation, has clear advantages over

dedicated ATR objectives or clip-on

devices. The spring-loaded micro-ATR

crystal provides a simple measure-

ment technique which does not risk

damage to either the crystal or the

cassegrain objective. The perfor-

mance of the PerkinElmer micro-

scopes permit the rapid collection

of high quality micro-ATR spectra

for a wide variety of samples, and

measurements can be extended to

include depth-profile studies.

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Figure 6. Micro-ATR depth profile spectra of a paper contaminant and bulk paper.Collection time: 20 sec each. Resolution: 8 cm -1.

Figure 7. Micro-ATR depth profile spectra from a fragment of an ancient wall painting.Collection time: 15 sec each. Resolution: 8 cm -1.

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